Making lightless connections in an optical circuit switch

ABSTRACT

Methods of operating an optical circuit switch and optical circuit switches are disclosed. A command to make an optical connection between a first port and a second port may be received, the first port uniquely associated with a first rotatable mirror element and the second port uniquely associated with a second rotatable mirror element. A determination whether or not input signal light is present at the first port may be made. When light is present at the first port, the first mirror element and the second mirror may be rotated to provide an initial optical connection between the first port and the second port. When light is not present at the first port, the first mirror element and the second mirror element may be placed in respective at-rest positions.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 13/958,872, filedAug. 5, 2013, titled MAKING LIGHTLESS CONNECTIONS IN AN OPTICAL CIRCUITSWITCH.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND

1. Field

This disclosure relates to optical communications networks and moreparticularly to optical circuit switches using MEMS(micro-electromechanical system) mirror arrays.

2. Description of the Related Art

Communications networks commonly contain a mesh of transmission pathswhich intersect at hubs or nodes. At least some of the nodes may includea switching device that receives data or information arriving at thenode and retransmits the information along appropriate outgoing paths.

Optical fiber links are commonly used to provide high bandwidthtransmission paths between nodes. Such optical fiber links form thebackbone of wide area networks such as the Internet. Optical fiber linksare also applied in high bandwidth local area networks which may beused, for example, to connect server racks in large data centers or toconnect processors in high performance computers.

An optical circuit switch is a switching device that forms connectionsbetween pairs of optical fiber communications paths. A typical opticalcircuit switch may have a plurality of ports and be capable ofselectively connecting any port to any other port in pairs. Since anoptical circuit switch does not convert information flowing over theoptical fiber communication paths to electrical signals, the bandwidthof an optical circuit switch is essentially the same as the bandwidth ofthe optical communications paths. Further, since an optical circuitswitch does not convert information into electrical signals, the powerconsumption of an optical circuit switch may be substantially lower thana comparable conventional (i.e. electronic) switch.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical schematic diagram of an optical circuit switch.

FIG. 2 is a block diagram of an environment for an optical circuitswitch.

FIG. 3 is a block diagram of an optical circuit switch.

FIG. 4 is a block diagram of a computing device.

FIG. 5 is a flow chart of a process for operating an optical circuitswitch.

FIG. 6A is a graph illustrating a position versus time characteristic ofa hypothetical mirror element in an optical circuit switch.

FIG. 6B is a graph illustrating a position versus time characteristic ofa hypothetical mirror element in an optical switch operated according tothe process of FIG. 5.

Throughout this description, elements appearing in figures are assignedthree-digit reference designators, where the most significant digit isthe figure number where the element is introduced and the two leastsignificant digits are specific to the element. An element that is notdescribed in conjunction with a figure may be presumed to have the samecharacteristics and function as a previously-described element havingthe same reference designator.

DETAILED DESCRIPTION

Referring now to FIG. 1, an exemplary optical circuit switch 100 may beconfigured to connect a group of n input ports (where n is an integergreater than 1), labeled In 1 to In n, to a group of n output ports,labeled Out 1 to Out n. More specifically, the optical circuit switch100 may selectively connect up to n pairs of ports, where each pair ofports includes an input port and an output port.

Each of the input ports In 1 to In n may include a connector (of whichonly the connector 110-1 is identified) to receive an input opticalsignal from a optical fiber cable (not shown) external to the opticalcircuit switch. Each connector may be coupled by a respective opticalfiber (of which only optical fiber 112-1 is identified) to a respectivetap coupler (of which only tap coupler 114-1 is identified). Each tapcoupler may extract an input sample portion, for example 1% to 10%, ofthe input optical signal from the respective optical fiber. Each inputsample portion may be directed to an input optical monitoring module170. The remainder of the input optical signals, other than the inputsample portions, may be conveyed by respective optical fibers torespective collimator lenses (of which only collimator lens 118-1 isidentified). Each collimator lens may convert the input optical signalfrom the respective optical fiber into a collimated input optical beam(of which only input optical beam 120-1 is identified) in free space.Free space optical beams are shown in FIG. 1 as dashed lines.

Each input optical beam, such as input optical beam 120-1, may bedirected onto a first mirror array 130. The first mirror array 130 mayinclude n mirrors with a one-to-one correspondence between input opticalbeams and mirrors, such that each input optical beam is directed onto arespective mirror. To improve the manufacturing yield of the firstmirror array, the first mirror array 130 may include more than nmirrors, in which case the n input optical beams may be directed to asubset of n mirrors that are known to be fully functional. Since each ofthe n input optical beams originates from a specific port and isdirected onto a specific mirror, each port may be described as “uniquelyassociated” with a corresponding mirror. In this patent, “uniquelyassociated” means a one-to-one correspondence. To take advantage of theavailable fully functional mirrors, the associations between ports andmirrors may be different in different optical circuit switches

Each mirror on the first mirror array 130 may reflect the respectiveinput optical beam to a selected mirror of a second mirror array 140.The mirrors of the second mirror array 140 may reflect the incident beamto form a respective output optical beam (of which only output opticalbeam 160-1 is identified). Each output optical beam may be directed to acorresponding focusing lens (of which only focusing lens 158-1 isidentified). Each focusing lens may focus the respective output opticalbeam into an output optical signal in a respective optical fiber. Eachoutput optical signal may be conveyed to a respective output tap coupler(of which only output tap coupler 154-1 is identified). Each output tapcoupler may direct a sample portion (for example 1% to 10%) of therespective output optical signal to an output optical monitoring module180. The remainder of each output optical signal, other that therespective sample portion, may be conveyed to a respective output portconnector (of which only connector 150-1 is identified).

The optical circuit switch 100 may create a one-to-one connectionbetween each input port and any one of the output port. For example, asshown in FIG. 1, Port In 1 is connected to port Out 2, port In 2 isconnected to port Out n, and port In n is connected to port Out 1.

The detail view 105 shows a simplified schematic diagram of a mirrorfrom either the first mirror array 130 or the second mirror array 140. Areflective mirror element 142 is supported by a pair of torsion bars, ofwhich only a first torsion bar 144 is visible. The second torsion bar islocated on the far side of the mirror element 142 and axially alignedwith the first torsion bar 144. The mirror element 142 may rotate aboutthe axis of the torsions bars, with the torsion bars providing a springforce tending to return the mirror element 142 to a default position.The mirror element 142 may be coupled to a first electrode 146 and asecond electrode 148. The mirror element 142 may be rotated byelectrostatic attraction between the mirror element and either the firstelectrode 146 or the second electrode 148.

For example, applying a voltage between the first electrode 146 and themirror element 142 will create an attraction between the mirror elementand the first electrode, causing the mirror element to rotate in acounter-clockwise direction. The mirror will rotate until the returnforce of the torsion bars is equal to the force of the electrostaticattraction. The angular rotation of the mirror element 142 may beapproximately proportional to the square of the voltage between thefirst electrode 146 and the mirror element 142. Similarly, applying avoltage between the second electrode 148 and the mirror element 142 willcause the mirror to rotate in a clockwise direction. The first electrode146 and the second electrode may be “dedicated” to the mirror element142, which is to say the only function of the electrodes 146 and 148 isto rotate the mirror element 142 and the voltages applied to theelectrodes 146 and 148 have no effect on any mirror element other thanthe mirror element 142.

In the simplified example of FIG. 1, the mirror element 142 rotatesabout a single axis defined by the torsion bars 144. Either or both ofthe first mirror array 130 and the second mirror array 140 may includemirrors configured to independently rotate about two orthogonal axes. Inthis case, each mirror element may be coupled to a first pair ofelectrodes to cause clockwise and counter-clockwise rotation about afirst axis and a second pair of electrodes to cause clockwise andcounter-clockwise rotation about a second axis orthogonal to the firstaxis. The structure of a mirror array and the associated electrodes maybe substantially more complex than that shown in the simplifiedschematic detail view 105. For example, U.S. Pat. No. 6,628,041describes a MEMS mirror array having two-axis mirror motion and combactuators.

The input optical monitoring module 170 and the output opticalmonitoring module 180 may be a common module. The input opticalmonitoring module 170 and the output optical monitoring module 180 maymeasure the optical power in each of the input sample portions andoutput sample portions, respectively. Each of the input opticalmonitoring module 170 and the output optical monitoring module 180 mayinclude an optical power detector for each sample portion.Alternatively, each of the input optical monitoring module 170 and theoutput optical monitoring module 180 may time-multiplex a singledetector or an array of detectors such that each detector measures theoptical power of sequence of sample portions. For example, each of theinput optical monitoring module 170 and the output optical monitoringmodule 180 may use a scanning mirror to direct sample portions to asingle detector or an array of detectors as described in U.S. Pat. No.7,676,125.

Sample portions may be extracted from the input optical beams, such asinput optical beam 120-1, and/or the output optical beams, such asoutput optical beam 160-1, using one or more free space sampling opticalelements. For example, sample portions may be extracted as described inU.S. Pat. No. 6,597,825 or U.S. Pat. No. 6,668,108. Input tap couplers,such as input tap coupler 114-1 and/or output tap couplers, such asoutput tap coupler 154-1, may not be present when free-space samplingoptical elements are used to extract sample portions.

Referring now to FIG. 2, an environment 295 for the application of anoptical circuit switch 200 may include a network 290 and a networkcontroller 210. The optical circuit switch 200, which may be the opticalcircuit switch 100, may be disposed within the network 290 and mayfunction to switch optical connections between other nodes (not shown)within the network 290. The network 290 may be, for example, a wide areanetwork, a local area network, a storage area network, a private networkwithin a data center or computer cluster, and may be or include theInternet. While the connections switched by the optical circuit switch200 are optical, other connections within the network 200 may be wiredand/or wireless.

The network controller 210 may be a computing device that provides agraphic user interface or a command line interface for a networkoperator to enter connection commands (i.e. commands to make or breakone or more optical connections) for the optical circuit switch 200. Thenetwork controller 210 may be a computing device running networkmanagement software, in which case connection commands for the opticalcircuit switch 200 may be generated automatically by the networkcontroller 210.

A communications link 215 between the optical circuit switch and thenetwork controller 210 may be in-band, which is to say thecommunications link 215 may be a path within the network 290. In thiscase, the optical circuit switch may have a wired, wireless, or opticalconnection to the network in addition to the optical connections beingswitched. The communications link 215 may be out-of-band, which is tosay the communications link 215 may be a dedicated connection or aconnection via a command network independent from the network 290. Aconfiguration in which the network controller 210 executes networkmanagement software to automatically provide connections commands to theoptical circuit switch 200 via an out-of-band command network 215 is anexample of what is commonly called a “software defined network”.

FIG. 3 is a high-level block diagram of the control and mirror driverportions of an optical circuit switch 300, which may be the opticalcircuit switch 100. The optical circuit switch 300 may include a switchcontroller 310, an input optical monitoring module 370, an outputoptical monitoring module 380, and a plurality of mirror driver circuits350. The optical circuit switch 300 may include one mirror drivercircuit 350 for each mirror in two mirror arrays if the individualmirror elements are rotatable about a single axis. The optical circuitswitch 300 may include two mirror driver circuits 350 for each mirror inthe mirror arrays if the individual mirror elements are rotatable abouttwo orthogonal axes. Each mirror driver circuit 350 may have, forexample, two selectable outputs to drive one or the other of a pair ofelectrodes, as described in pending patent application Ser. No.13/787,621.

The controller 310 may include a command interpreter 320 and a positionoptimizer 330 which jointly maintain a connection state table 340. Thecontroller 310 may receive connection commands from an external sourcesuch as the network controller 210. The controller 310 may receiveconnection commands from some other source or in some other manner.

The command interpreter 320 may be responsive to a set of connectioncommands received by the controller 310. The set of connection commandsmay include, for example “Break a-b” and “Make a-b”. These commands mayrespectively instruct the optical circuit switch 300 to either break anexisting connection between ports a and b (where a is an integer inputport number and b is an integer output port number), or to make a newconnection between ports a and b. The set of connection commands mayinclude a mass connection command, which may list multiple connectionsto be made. The mass connection command may be used, for example, whenthe optical circuit switch is initially integrated into a network orwhen substantial reconfiguration of the network or data center isrequired.

The command interpreter 320 may include or have access to a port map322. As previously described, to allow the use of mirror arrays with asmall number of nonoperational mirror elements, the number of mirrorelements in each mirror array may be larger than the number of input oroutput ports. Each input and output port may be coupled to a knownoperational mirror element in the respective mirror array. The port map322 may be a table containing data relating each input port to a mirrorelement in a first mirror array, and data relating each output port to amirror element in a second mirror array. The data in the port map 322may be specific to the particular first and second mirror arrays used inthe optical circuit switch 300.

There may be some performance variation from mirror element to mirrorelement and/or from mirror array to mirror array. In particular, theremay be some variation in the mirror element rotation angle versusapplied voltage characteristics within and between mirror arrays. Thecommand interpreter 320 may include or have access to a mirrorcalibration table 324 which contains data describing the performance ofeach mirror element. For example, the mirror calibration table 324 maystore the rotation angle versus voltage characteristic of each mirrorelement. The mirror calibration table 324 may store, for all possiblepairs of input and output mirror elements, a set of voltages that, whenapplied to the appropriate electrodes, will cause the mirror elements torotate to make the desired connection. The data in the mirrorcalibration table 324 may be specific to the particular mirror arraysused in the optical circuit switch 300. The data in the mirrorcalibration table 324 may be derived, for example, from the results oftests performed on the particular mirror arrays used in the opticalcircuit switch 300.

The data stored in the mirror calibration table 324 may indicate nominalvoltages required to initially make desired connections through theoptical circuit switch 300. However, once voltages are applied toelectrodes associated with a pair of input and output mirror elements toinitially make a connection, the positions of the mirror elements maydrift over time. The result of mirror element drift may be failure ordegradation (e.g. increased insertion loss) of the connection. Themirror arrays used in the optical circuit switch 300 may be fabricatedby chemical micromachining of a silicon substrate. For example, eachmirror element may consist of a reflective coating on a silicon slabthat is connected to the silicon substrate by narrow silicon elementsthat function as torsion bars. Each silicon mirror slab may be free torotate about the axis or axes defined by the torsion bars. Mirrorelement drift may be due to mechanical stress relief of the torsion barsover time. Further, all or portions of the silicon surfaces of themirror array may be coated with SiO2 or some other dielectric. Electriccharge trapped at defects in the insulators layers may contribute tomirror element drift over time. Other causes may also contribute tomirror element drift.

The position optimizer 330 may receive data from the input opticalmonitoring module 370 and the output optical monitoring module 380indicating the power levels at the input ports and the output ports,respectively. The position optimizer 330 may determine the insertionloss of each active optical connection (i.e. each optical connectionwhere light is present) from the respective input and output powerlevels. The position optimizer 330 may periodically adjust the positionsof some or all of the mirror elements to minimize the insertion loss ofeach optical connection. For example, to optimize a connection, theposition optimizer 330 may make incremental changes in the position ofone of the mirror elements used in the connection and observe theresulting effect on insertion loss. The optimum position of the mirrorelement may then be found using a hill climbing algorithm or a similaralgorithm. The position of each mirror element may be optimizedperiodically. The time interval between successive optimizations of eachmirror element may be short (on the order of seconds) compared to thetime constant of the mirror element drift (on the order of hours).Periodic optimization of the position of each mirror element mayautomatically compensate for mirror element drift.

The command interpreter 320 and the position optimizer 330 may jointlymaintain and share the connection state table 340. The connection statetable 340 may include data indicative of the state or status of eachport of the optical circuit switch 300. Data included in the connectionstate table 340 for each port may include a first flag indicating if therespective port is available or committed to a connection, and a secondflag indicating if the connection has actually been made. The connectionstate table 340 may include, for input ports, a third flag indicating islight is present at the respective input port. For each port that iscommitted to a connection, the connection state table 340 may alsoinclude the identity of the port at the other end of the connection, themirror element associated with the port, the voltages presently appliedto the electrodes associated with the mirror element, and temporal datasuch as when the connection was first made and when the position of themirror element was most recently optimized.

Referring now to FIG. 4, a switch controller 400, which may be suitablefor use as the switch controller 310 in the optical circuit switch 300,may include a processor 410 coupled to a memory 450 and input/outputinterfaces 460. The switch controller 400 may implement the functions ofthe command interpreter 320 and the position optimizer 330.

The processor 410 may include one or more processing devices, such asmicroprocessors and programmable signal processors, that executesoftware instructions stored in the memory 450. The processor 410 mayinclude one or more non-programmable processing devices that executehardware-defined functions. The processor 410 may include otherspecialized circuits and devices as required to implement the methodsand functions described herein. When the processor 410 includes multipleprocessing devices, the processing devices may be collectively orseparately coupled to the memory 450 and the input/output interfaces460.

The memory 450 may include both nonvolatile memory and volatile memory.The nonvolatile memory may be or include semiconductor read-only memory,semiconductor nonvolatile memory such as flash memory, magnetic storagedevices such as magnetic disc drives, and optical storage devices suchas CD-ROM or CD-R/W drives. The volatile memory may be static or dynamicsemiconductor random access memory (RAM).

The memory 450 may store software instructions for execution by theprocessor 410. The software instructions may include programinstructions 452 and an operating system 454. The program instructions452 may include instructions to cause the processor 410 to implement allor part of the functions of the command interpreter 320 and the positionoptimizer 330. The operating system 454 may be, for example, a versionof Microsoft Windows®, Unix®, Linux®, Google Chromium®, Apple® MAC OS,or some other operating system.

The memory 450 may store tables, lists, and other data for use by theprocessor 410. The data stored in the memory 450 may include a port map422 that identifies which mirror on one of two mirror arrays isassociated with each port of the optical circuit switch. The data storedin the memory 450 may include a mirror calibration table 424 whichcontains data describing the performance of each movable mirror in theoptical circuit switch. The data stored in the memory 450 may include aconnection state table 440 listing a status of each port, mirror, andconnection and associated information.

The input/output interfaces 460 may include circuits, devices, and/orfirmware to transfer data between the processor 410 and one or moreoptical monitoring modules (OMMs), a large plurality of mirror drivers,and a connection to a network controller such as the network controller210.

Description of Processes

Referring now to FIG. 5, a process 500 to make a connection through anoptical circuit switch, such as the optical circuit switch 100, maystart at 505 and end at 595. The process 500 may be repeated numeroustimes during the operation of an optical circuit switch to make acorresponding number of connections. The process 500 may be executed bya controller, such as the controller 310 or the controller 400.

At 510, a Make command may be received, for example, from a networkcontroller such as the network controller 210. The Make command mayidentify one or more connections to be made through the optical circuitswitch. For each connection, the Make command may specify an input portand an output port to be connected. For ease of discussion, thesubsequent description of the process 500 will be directed to making asingle connection, with the understanding that the actions of theprocess 500 may performed, sequentially or concurrently, for each of aplurality of connections.

At 520, a determination may be made if the input and output portsidentified in the Make command are available. For example, adetermination may be made whether or not both the input port and theoutput ports are listed as uncommitted in the connection state table340. If one or both ports are not listed as uncommitted in theconnection state table 340 (“no” at 520), the command may be rejected at525, and the process 500 may end at 595. The command may be rejected at525, for example, by displaying or sending a message indicating that oneor both of the ports is not available. Mirror elements associated withavailable ports may be disposed in respective at-rest positions.

When both the input port and output port identified in the Make commandreceived at 510 are available (“yes” at 520), the requested connectionmay be provisioned at 530. To provision the connection at 530, theconnection state table may be changed to indicate the input port and theoutput port identified in the Make command are committed to therequested connection and are no longer available for other connections.At 535, the mirror elements associated with the input port and theoutput port may be identified, for example by consulting the port map322.

As previously described, mirror elements are subject to drift. Whenvoltages are applied to electrodes associated with a mirror element, themirror element may rotate initially to a desired position. Subsequently,the mirror element position may drift over time due to a combination ofmechanical and electrical effects. To compensate for mirror elementdrift, an optical circuit switch may include a position optimizer, suchas the position optimizer 330, to periodically adjust the position ofeach mirror element used in a connection to minimize the insertion lossof each connection. However, the function of the position optimizer mayrely upon measurements of the input and/or output optical power for eachconnection. Mirror element position optimization, and thus driftcompensation, may not be possible for dark or lightless connectionswhere there is no input and output light to be measured.

FIG. 6A illustrates a problem that may occur in a lightless connection.The lower chart plots the voltage applied to a mirror electrode versustime, and the upper chart plots the mirror position on the same timescale. A connection is made at a first time point T1, causing a nominalvoltage (Vnom) to be applied to the electrode. As a result, the mirrorelement moves from a resting position (Rest) to a required (in order tomake the defined connection) position (Req). In the subsequent timeperiod Pa, the mirror element position may drift away from the requiredposition. Since light is not present in the connection, the drift cannotbe detected or compensated. Thus the applied voltage remains constantduring time period Pa.

At the end of time interval Ta, at time T2, light is transmitted throughthe optical connection. However, by this time the connection may havedegraded or may be broken due to the accumulated mirror element positiondrift. During time period Pb, the position optimizer may search for anappropriate voltage that optimizes the connection. The search may beperformed, for example, using a hill climbing algorithm or similarsearch algorithm. During time period Pb, as the search is performed, theapplied voltage and the mirror element position may oscillate and theconnection may be unreliable. The length of the time period Pb willdepend on the magnitude of the accumulated position drift, which will inturn depend on the magnitude of the applied voltage and the duration ofthe time period Pa. Eventually, the correct voltage may be determinedand applied to the electrodes associated with the mirror element, andthe connection may be optimized. During time period Pc, the connectionis established and any further mirror element position drift may becompensated automatically by the position optimizer.

FIG. 6B illustrates the operation of an optical circuit switch using theprocess 500. The lower chart plots the voltage applied to a mirrorelectrode versus time, and the upper chart plots the mirror position onthe same time scale. A connection is defined at a first time point T3.At T3, the connection may be provisioned, which is to say the mirrorelement may be committed, but no voltage is applied to the associatedelectrodes during time period Pd. At time T4, light is transmittedthrough the optical connection. The nominal voltage (Vnom) is nowapplied to the electrode, causing the mirror element to move to therequired position over a time period Pe, which may be much shorter thantime period Pb of FIG. 6A. Subsequently, mirror element position driftduring the time period Pf may be automatically compensated by theposition optimizer, which may adjust the applied voltage to maintain theoptimum mirror element position.

Referring back to FIG. 5, at 540 a determination may be made if inputsignal light is present at the input port to be connected. Thisdetermination may be made, for example, by consulting the connectionstate table 340, which may include flags indicating whether or not lightis present at each input port. These flags may be set based on inputoptical power measurements made by the input optical monitoring module370. When a determination is made at 540 that input signal light is notpresent at the input port to be connected, the mirrors identified at 535may be placed in respective at-rest positions at 565. The process 500may loop continuously between 540 and 565 until input signal lightbecomes present or until the mirror elements are de-provisioned by someother process (not shown).

When a determination is made at 540 that input signal light is presentat the input port to be connected, the mirror elements to be used in theconnection may be rotated at 545 to establish an initial connectionbetween the input port and the output port. Rotating the mirrors at 545may include applying nominal voltages to electrodes associated with themirror elements. The nominal voltages may be determined, for example, byconsulting the mirror calibration table 324. The nominal voltages may beapplied gradually to minimize or prevent the mirror elements ringing orundergoing position oscillations.

After the initial connection is established at 545, the positionoptimizer may assume control of the mirror elements and optimize arotation angle of one or both the mirror elements at 550. Optimizing therotation angles of the mirror elements at 550 may occur periodicallyuntil a determination is made at 555 that the light in the connectionhas been lost or disrupted, or until a determination is made at 560 thata command to break the connection has been received. The actions at 550,555, and 560 are shown as sequential for ease of explanation but may beperformed concurrently.

A determination that the light is lost may be made at 555 if, forexample, there is no light, or less than a predetermined optical power,measured by the input optical monitoring module. To avoid responding totransient conditions on the network, a determination that the light islost may be made only if the absence of light persists for more than apredetermined time interval. The predetermined time interval may beselected to be longer than any routine transient interruption of thelight. When a determination is made at 555 that the light is lost, themirror elements may be returned to their respective at-rest positions at565 and the process 500 may return to 540 to await the return of lightat the input port. Returning the mirror elements to the respectiveat-rest positions may include setting the voltage on the associatedelectrodes to zero volts or some other default value. Mirror elementsplaced in their at-rest positions at 565 may nonetheless remaincommitted to the connection until a break connection command isreceived.

When a determination is made at 560 that a command to break theconnection has been received, the mirror elements may be returned totheir respective at-rest positions at 570. In this situation, theconnection may be de-provisioned at 580 by releasing the input port andthe output port for use in other connections. Releasing the ports may beperformed by appropriately setting flags in the connection state table340 to indicate that the ports are available for use in anotherconnection. The process 500 may then end at 595.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A method for making a connection in an optical circuitswitch, comprising: receiving a command to make an optical connectionbetween a first port and a second port, the first port uniquelyassociated with a rotatable first mirror element and the second portuniquely associated with a rotatable second mirror element; determiningwhether or not input signal light is present at the first port; whenlight is present at the first port, rotating the first mirror elementand the second mirror element to provide an initial optical connectionbetween the first port and the second port; and when light is notpresent at the first port, waiting until light becomes present at thefirst port before rotating the first mirror element and the secondmirror element to provide the initial optical connection.
 2. The methodof claim 1, wherein the first mirror element and the second mirrorelement are disposed in respective at-rest positions at the time ofreceiving the command to make the optical connection between the firstport and the second port, and the method further comprises maintainingthe first mirror element and the second mirror element in theirrespective at-rest position while waiting for light to become present atthe first port.
 3. The method of claim 1, further comprising: afterrotating the first mirror element and the second mirror element toprovide the initial optical connection, determining whether or not inputsignal light is still present at the first port; and if optical signallight is no longer present at the first port, returning the first mirrorelement and the second mirror element to respective at-rest positions.4. The method of claim 3, further comprising: after returning the firstmirror element and the second mirror element to the respective at-restpositions, determining whether or not input signal light is againpresent at the first port; and if input signal light is again present atthe first port, rotating the first mirror element and the second mirrorto reestablish the initial optical connection between the first port andthe second port.
 5. The method of claim 1, further comprising: afterrotating the first mirror element and the second mirror element toprovide the initial optical connection, adjusting a rotation angle of atleast one of the first mirror element and the second mirror element toreduce an insertion loss of the optical connection.
 6. The method ofclaim 1, further comprising: after receiving the command to make anoptical connection between the first port and the second port,provisioning the connection by making the first port and the second portunavailable for any other connection, independent of the presence ofinput signal light at the first port.
 7. The method of claim 6, furthercomprising: after rotating the first mirror element and the secondmirror element to provide the initial optical connection, determining ifa command has been received to break the optical connection between thefirst port and the second port; and when a command has been received tobreak the optical connection between the first port and the second port:returning the first mirror element and the second mirror element torespective at-rest positions, and de-provisioning the connection betweenthe first port and the second port, making the first port and the secondport available for other connections.
 8. The method of claim 1, furthercomprising: upon receiving the command to make the optical connectionbetween the first port and the second port, determining whether or notboth the first port and the second port are both available; and if atleast one of the first port and the second port are not available,rejecting the command without performing any other actions of themethod.
 9. An optical circuit switch, comprising: a first port uniquelyassociated with a first rotatable mirror element; a second port uniquelyassociated with a second rotatable mirror element; a controllerconfigured to: receive a command to make an optical connection betweenthe first port and the second port, determine whether or not inputsignal light is present at the first port; when light is present at thefirst port, cause the first mirror element and the second mirror torotate to provide an initial optical connection between the first portand the second port; and when light is not present at the first port,waiting until light becomes present at the first port before causing thefirst mirror element and the second mirror to rotate to provide theinitial optical connection.
 10. The optical circuit switch of claim 9,wherein the first mirror element and the second mirror element aredisposed in respective at-rest positions at the time of receiving thecommand to make the optical connection between the first port and thesecond port, and the controller is further configured to maintain thefirst mirror element and the second mirror element in their respectiveat-rest position while waiting for light to become present at the firstport.
 11. The optical circuit switch of claim 9, wherein the controlleris further configured to: after causing the first mirror element and thesecond mirror element to rotate to provide the initial opticalconnection, determine whether or not input signal light is still presentat the first port; and if optical signal light is no longer present atthe first port, cause the first mirror element and the second mirrorelement to return to the respective at-rest positions.
 12. The opticalcircuit switch of claim 11, wherein the controller is further configuredto: after causing the first element and the second mirror element toreturn to the respective at-rest positions, determine whether or notinput signal light is again present at the first port; and if opticalsignal light is again present at the first port, cause the first mirrorelement and the second mirror element to rotate to reestablish theinitial optical connection between the first port and the second port.13. The optical circuit switch of claim 9, wherein the controller isfurther configured to: after causing the first mirror element and thesecond mirror element to rotate to provide the initial opticalconnection, adjust a rotation angle of at least one of the first mirrorelement and the second mirror element to reduce an insertion loss of theoptical connection.
 14. The optical circuit switch of claim 9, whereinthe controller is further configured to: after receiving the command tomake an optical connection between the first port and the second port,provision the connection by making the first port and second portunavailable for any other connection, independent of the presence ofinput signal light at the first port.
 15. The optical circuit switch ofclaim 14, wherein the controller is further configured to: afterrotating the first mirror element and the second mirror element toprovide the initial optical connection, determine if a command has beenreceived to break the optical connection between the first port and thesecond port; and when a command has been received to break the opticalconnection between the first port and the second port: cause the firstmirror element and the second mirror element to return to the respectiveat-rest positions, and de-provision the connection between the firstport and the second port, making the first port and the second portavailable for other connections.
 16. The optical circuit switch of claim9, wherein the controller is further configured to: upon receiving acommand to make the optical connection between the first port and thesecond port, determine whether or not the first port and the second portare available; and if at least one of the first port and the second portare not available, reject the command without performing any otheractions.
 17. The optical circuit switch of claim 9, further comprising:a plurality of input ports including the first port, each of theplurality of input ports uniquely associated with a respective one of aplurality of rotatable mirror elements, including the first mirrorelement, in an input mirror array; and a plurality of output portsincluding the second port, each of the plurality of output portsuniquely associated with a respective one of a plurality of rotatablemirror elements, including the second mirror element, in an outputmirror array.
 18. A method for making connections in an optical circuitswitch, comprising: receiving a command to make one or more opticalconnections, each optical connection between an input port from aplurality of input ports and an output port from a plurality of outputports, each of the input ports and the output ports uniquely associatedwith a respective rotatable mirror element; and for each of the one ormore optical connections: determining whether or not input signal lightis present at the respective input port, when light is present at therespective input port, rotating the mirror elements associated with therespective input port and the respective output port to provide aninitial optical connection between the respective input port and therespective output port, and when light is not present at the respectiveinput port, waiting until light becomes present at the respective inputport before rotating the mirror elements associated with the respectiveinput port and the respective output port to provide the initial opticalconnection.
 19. The method of claim 18, wherein, for each of the one ormore optical connections: the mirror elements associated with therespective input port and the respective output port are disposed inrespective at-rest positions at the time of receiving the command tomake the optical connection between the input port and the output port,and the method further comprises maintaining the mirror elementsassociated with the respective input port and the respective output portin their respective at-rest position while waiting for light to becomepresent at the input port.
 20. The method of claim 18, furthercomprising, for each of the one or more optical connections: afterrotating the mirror elements associated with the respective input portand the respective output port to provide the initial opticalconnection, determining whether or not input signal light is stillpresent at the respective input port; and if optical signal light is nolonger present at the respective input port, returning the mirrorelements associated with the respective input port and the respectiveoutput port to the respective at-rest positions.
 21. The method of claim20, further comprising, for each of the one or more optical connections:after returning the mirror elements associated with the respective inputport and the respective output port to the respective at-rest positions,determining whether or not input signal light is again present at therespective input port; and if optical signal light is again present atthe respective input port, rotating the mirror elements associated withthe respective input port and the respective output port to reestablishthe initial optical connection between the respective input port and therespective output port.
 22. The method of claim 18, further comprising,for each of the one or more optical connections: after rotating themirror elements associated with the respective input port and therespective output port to establish the initial optical connection,adjusting a rotation angle of at least one of the mirror elementsassociated with the respective input port and the respective output portto reduce an insertion loss of the optical connection.
 23. The method ofclaim 18, further comprising, for each of the one or more opticalconnections: after receiving the command to make the optical connectionbetween the respective input port and the respective output port,provisioning the connection by making the respective input port and therespective output port unavailable for any other connection, independentof the presence of input signal light at the respective input port. 24.The method of claim 23, further comprising, for each of the one or moreoptical connections: after rotating the mirror elements associated withthe respective input port and the respective output port to establishthe initial optical connection, determining if a command has beenreceived to break the optical connection; and if a command has beenreceived to break the optical connection: returning the mirror elementsassociated with the respective input port and the respective output portto the respective at-rest positions, and de-provisioning the connection,making the respective input port and the respective output portavailable for other connections.
 25. The method of claim 18, furthercomprising, for each of the one or more optical connections: uponreceiving the command to make the optical connection between therespective input port and the respective output port, determiningwhether or not the respective input port and the respective output portare available; and if at least one of the respective input port and therespective output port is not available, rejecting the command withoutperforming any other actions of the method.